1. Field of the Invention
The present invention relates to a light-emitting diode and a method of fabricating the same, and more particularly, to a light-emitting diode to which zinc oxide (ZnO) nanorods are applied and a method of fabricating the same.
2. Description of the Related Art
GaN-based light-emitting diodes (LEDs) have recently attracted much attention in the field of light-emitting devices, communication devices, and large displays. Especially, their inherent properties such as miniaturization, high brightness, reliable long-lifetime, short response time, and low energy consumption provide significant advantages over existing light-emitting devices.
Nevertheless, the external quantum efficiency (EQE) of the GaN-based LEDs, which is closely associated with their internal quantum efficiency (IQE) and light extraction efficiency (LEE), is still quite low in typical InxGa1-xN/GaN quantum well (QW) structures. One of the primary reasons for the low EQE results from the low light extraction efficiency, which is mainly caused by light loss due to the total internal reflection (TIR) occurring at the interface between the structure of the LED and the air or encapsulating material. The refractive indices of p-GaN, indium-tin-oxide (ITO), and air are 2.52, 2.06, and 1, respectively, and their critical angles (θcrit) are based on Snell's Law:θcrit=sin−1(n1/n2)
wherein n2 is the refractive index of the less optically dense medium and n1 is the refractive index of the more optically dense medium.
According to the previous research, the initial critical angles in the LEDs are 54.83° at the interface of p-GaN and ITO and 29.04° at the interface of ITO and air. Photons emitted over these critical angles can be reflected from and reabsorbed at the interface and internally confined. Thus, the improvement of the light extraction efficiency can be achieved by incorporating geometric structures in the LED to reduce internal reflection and increase light extraction at the interface. For example, the surface of p-GaN and/or ITO contact layer is roughened by an etching process, or a periodic pattern including a photonic crystal structure is fabricated by laser holographic lithography, e-beam lithography, and nanoimprint technology to improve the light extraction efficiency. However, it was found that the plasma process induced damage to the surface during the process, which, in turn, caused deterioration in electrical properties.
Recently, ZnO has been introduced as one of the most promising materials for short wavelength LEDs and laser diode applications using its inherent wide and direct bandgap of 3.37 eV. In addition, ZnO films have higher transparency and lower cost than ITO films. However, the application of ZnO to homojunction diodes is limited because of difficulties in fabricating a high quality ZnO film and conducting p-type doping on the ZnO film due to its intrinsic n-type properties. As an alternative approach to the use of the ZnO for optoelectronic applications, a ZnO/GaN heterojunction structure has been suggested. However, these heterojunction diodes have low efficiency due to high energy barrier at the junction interface. Another approach to improve the light extraction efficiency is to form an LED using ZnO nanorods on a ZnO transparent conductive layer. In this case, although improvement of light extraction efficiency is achieved, the LED having the ZnO nanorods exhibits low current spreading and electrical performance compared to typical ITO-based LEDs.